It is generally known that there are considerable problems involved in the application of blooming layers onto transparent plastic substrates as well as the preparation of antireflection coatings.
In appplicant's copending U.S. application Ser. No. 150,239 (now abandoned) it was determined in this context that the thermal expansion of plastics is about 200 times larger than the thermal expansion of conventional coating materials. It was further determined that even after the most careful purification in vacuum thin water skins or other traces remain on the surfaces of the substrates to be coated, which skins are only several atomic layers thick.
On the other hand it was found in German patent application De-OS No. 26 58 417 that the polydiethylene-glycol-diallyl-carbonate designated as CR-39 is very desirable as a transparent substrate for optical articles based on its optical properties and its relative hardness.
For this purpose in the above-mentioned application it was disclosed that in this case there are numerous problems also in the selection of suitable evaporating materials and that at least a double layer has to be constructed comprising a high-refractive index material and a lower-refractive index material in order to meet all requirements which have to be imposed on such blooming layers. In this context it has to be noted that the optical waves reflected at the air-evaporated-layer/substrate boundary do extinguish by interference. First, the layer has to meet the so-called index condition, which prescribes the index of refraction of the layer and which endures that the amplitudes of the two waves become the same. Second, the layer has to meet the phase condition, which requires a layer of a certain thickness so that the phase inversion occurs, which is required for the mutual extinction. It is a further requirement that these layers have to be hard and they have to hold up well and they should be well attached. In addition, the layers should be resistant to mechanical wear and to atmospheric influences. Finally, the layers have to be resistant to aging and are not to react with the oxygen of the air, since otherwise stains might form.
In order to meet these requirements the above-mentioned application teaches initially to treat the substrate surfaces to be bloomed with an electron beam and then to coat the thus cleaned surface with a first layer of silicon monoxide, a second layer of hafnium oxide and a third layer of silica.
These procedures have achieved coating qualities which had not been reached previously. However, such coated materials were not able to meet optimally all conditions to be imposed on high-use eye-glasses or watch glasses.